1. Field of the Invention
The present invention relates to a toner for use in electrophotography, an image forming method for visualizing an electrostatic charge image and a toner jet.
2. Description of the Related Art
In order to reduce the power consumption and shorten the wait time of an electrophotographic apparatus, an on-demand type fixing apparatus obtained by combining a ceramic heater with a small thermal capacity and a film has been put to practical use as a fixing apparatus. In such a fixing apparatus, attempts have been made to reduce a fixing nip internal pressure thereof from the viewpoint of elongation of the lifetime and applicability to a variety of media.
Furthermore, according to recent increase in a print speed, time when a toner and a medium such as paper pass through a nip of a fixing apparatus has become shorter year by year.
Moreover, there are recently increasing opportunities for a user to output, by using an image forming apparatus such as a laser beam printer (LBP), a graphic image with a high coverage rate such as image data taken through a digital camera, a portable terminal or the like, or a poster.
In this context, there is a demand for a toner capable of showing an excellent low temperature fixing property even under severer fixing conditions, for example, conditions for forming an image with a high coverage rate in a short period of time with a low nip internal fixing pressure. For purpose of meeting such conditions, a large number of toners using a crystalline resin as a binding resin have been proposed.
Japanese Patent Application Laid-Open No. 2010-102058 describes that a crystalline resin is rapidly molten at a temperature in the vicinity of glass transition temperature thereof, and that a low temperature fixing property of a toner can be improved by increasing the compatibility between a crystalline resin and an amorphous resin. When the compatibility therebetween is too high, however, the resulting toner is degraded in the heat-resistant storage property and the crystallizability on the contrary.
In contrast, when the compatibility between an amorphous resin and a crystalline resin is lowered, the crystal of the crystalline resin is liable to be easily formed, but these resins are hard to be compatible with each other even at a temperature over melting points thereof, and hence, it is difficult to improve the low temperature fixing property particularly when a fixing time is short or a nip internal pressure is low.
Japanese Patent Application Laid-Open No. 2010-152102 describes that recrystallization of a crystalline resin is accelerated by additionally performing a step of conducting a heat treatment at a specific temperature in a production process for a toner. When the method described in Japanese Patent Application Laid-Open No. 2010-152102 is employed, a toner containing crystal of the crystalline resin can be definitely obtained. If the toner is once molted in a fixing step, however, the crystalline resin and an amorphous resin are completely compatibilized and can never be restored to an original crystalline state, and therefore, a toner image obtained after fixing may be blocked under a severe environment of high temperature and high humidity.
Meanwhile, a method for accelerating crystallization of a crystalline resin by adding a crystal nucleating agent to a toner has been proposed. Japanese Patent Application Laid-Open No. 2007-033773 describes use of an inorganic crystal nucleating agent of fine particles of silica or the like as the crystal nucleating agent. Also, Japanese Patent Application Laid-Open No. 2006-113473 describes use of an organic crystal nucleating agent of a metal benzoate, fatty amide or the like as the crystal nucleating agent. If the content of an inorganic crystal nucleating agent of silica or the like is large, however, the crystal nucleating agent may show a filler effect to increase the melt viscosity of a resulting toner, which may inhibit the low temperature fixing property. Besides, an inorganic crystal nucleating agent affects a charging characteristic of the toner, and hence, a charging property of the toner may be difficult to control in many cases. On the other hand, an organic nucleating agent is a low molecular compound such as a metal benzoate or a fatty acid metal salt in many cases, and such a nucleating agent is segregated on the surface of the toner and shows an insufficient effect as the crystal nucleating agent. As a result, the storage property of the toner may be degraded or the storage stability of a toner image may be degraded in some cases.
Moreover, even though a part of crystal is recrystallized after fixing, the crystal is in an incomplete crystalline state. Therefore, if the fixed image is left to stand for a long period of time, phase separation is caused between a crystal portion and a compatible portion, and due to volume shrinkage occurring in the crystal portion, the fixed image may curl in some cases.
In this manner, there still remains a room for improvement for attaining both excellent low temperature fixing performance and long-term storage stability of a fixed image.